1. Field
The present invention relates to transducers of electrical energy to acoustic energy and vice versa. More particularly, the present invention relates to an apparatus for acoustic loading of a diaphragm in such a transducer.
2. Background
Transducers have been developed to allow transformation of electrical energy into acoustic energy, as well as to allow transformation of acoustic energy into electrical energy. The former are known as loudspeaker drivers, the latter are known as microphones.
Among the many considerations when designing such transducers is efficiency of energy transformation. To accomplish such an energy transformation, a transducer typically comprises a moving element—a diaphragm. In a driver, the moving element is responsive to the electrical energy. In a microphone, the moving element is responsive to the acoustic energy. Because of different properties, e.g., masses, between the media through which the acoustic energy propagates, e.g., air, and the moving element of a transducer, i.e., the diaphragm, the energy transformation efficiency is small.
To improve the energy transformation efficiency, designers construct the diaphragm from light materials in an attempt to minimize the diaphragm's mass, thus decreasing the mismatch of the properties. However, this design approach is limited by a multiplicity of factors, including, but not being limited to: properties of the diaphragm material, properties of the diaphragm structure, acoustic loading on the diaphragm, and frequency of operation. Even advanced materials used for diaphragm construction reach a point, beyond which further attempts to decrease the diaphragm's mass causes the diaphragm to change its shape during operation, which results in non-pistonic movement and; consequently, an erratic acoustic or electrical energy output. The term pistonic means that each point at the diaphragm keeps constant position relative to the other points at the diaphragm, as the diaphragm moves.
Another technique to improve the energy transformation efficiency used in the art is to increase acoustic loading, or radiation resistance, acting upon the diaphragm. This loading is achieved by placing an acoustic transformer in front of the diaphragm.
One known class of acoustic transformers comprises horns. In general, a horn is a device, which achieves the acoustic transformation by converting large pressure variations in a small amount of air into a low pressure variation in a large amount of air. The conversion is effected by increasing cross-section area in the progression from a throat of the horn to the mouth of the horn, according to a function describing the horn's flare. Modern horn designs typically feature some form of flare, e.g., exponential, tractrix, or conical.
Very often, the loading is further modified by making the cross-section area of the horn throat smaller than the area of the diaphragm of the transducer to which the horn is attached. The relationship of the horn throat cross-section area to the diaphragm area is typically referred to as a compression ratio of the horn.
Although the above-described horn improves efficiency of the energy transformation, the acoustic energy being radiated from different locations on the diaphragm in a form of sound waves, may arrive in the horn throat at different times due to differing path lengths. Such an arrival creates an out of phase condition causing irregularities in the frequency response.
A solution to this problem is to make paths from different parts of the diaphragm as similar as possible, to avoid phase cancellation that results from such an out of phase condition. One such a solution utilizes an acoustic transformer known as a phase—or phasing—plug, which is interposed between the diaphragm and the horn. A phasing plug thus improves loading of a diaphragm, and equalizes path lengths.
Reference is now made to FIG. 1, which illustrates a transducer in accordance with a general construction principles of prior art. Although FIG. 1 illustrates a driver, such is for tutorial purposes only because the below described concepts are equally applicable to microphones.
The transducer comprises an acoustic transformer, i.e., a phasing plug 102. The acoustic transformer possesses rotation symmetry about an axis 100. The axis of rotation is a line such that for every point of the body its distance to the line remains constant under the rotation, and the point remains in the same plane perpendicular to the axis. Thus the point moves in a circle in that plane.
A rear face 106 of the phasing plug 102 is shaped in accordance with the shape of a diaphragm 108, generally in a shape of a dome, or a portion of a sphere. To enable movement of the diaphragm 108, a surround 124 is attached in vicinity to the circumference of the diaphragm 108. The surround 124 operates like a suspension as well as locating device for the diaphragm 108, and is affixed to a first pole piece 110, by fasteners 112.
Because an acoustic capacitance of volume of air between the diaphragm 108 and the phasing plug 102 causes loss of high frequency energy, the clearance between the rear face of the phasing plug 102 and the diaphragm 108 is generally defined to allow only enough room for the diaphragm to move through the diaphragm's intended range without physical interference with the rear face. Such an arrangement minimizes the volume of air between the diaphragm and the phasing plug and; consequently, the acoustic capacitance.
The side of the diaphragm 108 facing the rear side of the phasing plug 102 is a compression side of the diaphragm. The non-compression side of the diaphragm 108 is protected by a cover 126, either sealed or vented.
To enable movement of the diaphragm 108 and; consequently, a transformation of electrical energy to acoustic energy and vice versa, a voice coil 122 is affixed in vicinity to the circumference of the compression side of the diaphragm 108.
A static magnetic flux is provided so that an alternating input signal causing a current flow through the voice coil 122, causes the voice coil 122 to move back and forth along the axis 100. A magnetic circuit, i.e., a closed path containing the static magnetic flux, comprises the first pole piece 110, a second pole piece 116, and pieces 118 and 119. The materials and methods of mechanical connection of the pieces 110, 116, 118 and 119, forming the magnetic circuit are designed to provide a low reluctance path for the static magnetic flux through the magnetic circuit.
The static magnetic flux in the magnetic circuit is induced by a magnet 120, a coil, or any other suitable means. The static magnetic flux produces a magnetic flux density in the air gap between the first pole piece 110 and the second pole piece 116.
A plurality of voids 114 between the rear face 106 and the front face 104 form air channels, allowing the sound waves to travel through the voids 114 from rear to front, and generally emerge at the front face 104 of the phasing plug 102 as a single air channel.
To improve the loading of the diaphragm 108, the total cross-section area of the air channels of the phasing plug 102 at the rear face 108 is made smaller than the total area of the diaphragm 108. The relationship of the total diaphragm area to the total cross-section area of the air channels is typically referred to as a compression ratio of the transducer. The air between the diaphragm and the phasing plug (i.e., the compression region), can be compressed to relatively high pressures by small motion of the diaphragm. This is what allows such a transducer to output acoustic energy at greater pressure levels than can conventional loudspeakers where the diaphragm radiates directly into the air. The efficiency of the transducer is thus increased. A transducer with such an arrangement is generally referred to as a compression driver.
Further to providing a compression ratio, the path lengths of the air channels within the phasing plug may be equalized so as to bring all portions of the sound wave, propagating through the air channels, into phase coherence when they reach the front face of the phasing plug. Without such path length equalization, sound waves emanating from different air channels would destructively combine so as to cause irregularities in the frequency response as discussed above.
The exit path of the transducer is bored into a second pole piece 116. The area of the front face 104 and the area of a transducer's exit 128 together with the distance between them define a flare. This flare may affect the useful frequency bandwidth of the transducer.
There are many designs of phasing plugs, accomplishing the compression loading and path length equalization. Perhaps the most frequently used type is a circumferential phasing plug. Such a phasing plug comprises annular cross-sections that usually increase in area as the principal radius of each annulus decreases in moving toward the throat of the transducer. An example of such a phasing plug can be found in U.S. Pat. No. 2,037,187, entitled “Sound Translating Device,” incorporated by reference. An often cited disadvantage of these phasing plugs—difficult and expensive manufacturing—lead to development of a radial phasing plug.
A radial phasing plug comprises a plurality of radial slot-shaped inlets extending from the axis of cylindrical symmetry of the speaker. An example of such a phasing plug can be found in U.S. Pat. No. 4,050,541, entitled “Acoustical Transformer for Horn-type Loudspeaker,” incorporated by reference.
Yet another type is a saltshaker design, so called because holes at the spherical outer surface of the plug that extend through to the throat of the speaker resemble the holes of a saltshaker. An example of a saltshaker phasing plug may be found in Fancher M. Murray: “An Application of Bob Smith's Phasing Plug,” 61st Convention of an Audio Engineering Society (AES), Nov. 3-6, 1978, New York.
However, all the known phasing plug designs suffer from several problems. Considering their design, the magnet 120, the first pole piece 110, and the second pole piece 116 are commonly located on the front side of the phasing plug 102. The voice coil 122 is disposed within the air gap between the first pole piece 110 and the second pole piece 116.
Ideally, the air gap should be made as narrow as is practicable since reluctance in the magnetic circuit increases as a square function of the width of the gap, lowering the magnetic flux density in the air gap rapidly as the dimension is increased. Nevertheless, there is a region, comprising a considerable volume of air in the air gap surrounding the voice coil 122 as well as in the spaces along the inner circumference of the surround 124 and outer circumference of the diaphragm 108. Because this region is far from the inlets of the phasing plug air channels, the variations of air pressure in that region are coupled negligibly, i.e., little or not at all, to the phasing plug 102 and; consequently the transducer's exit 128. As such the pressure variations do not contribute to the generation of sound output, and cause energy losses in the form of heat.
In addition, the uncoupled region also causes cavity resonance effects which distort the overall sound output of the speaker due to anomalies in its frequency response. The problem is treated in, e.g., Kinoshita, et al.: “The Influence of Parasitic Resonances on Compression Driver Loudspeaker Performance”, 61st Convention of the Audio Engineering Society in 1978.
The ideal behavior of a diaphragm movement would be a purely pistonic motion over the entire area of the diaphragm in response to forces imposed upon it by the input signal, over the entire range of audio frequencies being reproduced by the transducer. However, this is and ideal, and can not be achieved in practice. In general, above a certain audio frequency, the diaphragm begins to deform, and portions of the diaphragm move non-pistonically. This deformation results in creation of signals not present in the input signal (a distortion).
As the frequency increases, the properties of the diaphragm, e.g., the mass and lack of stiffness, cause region(s) of the diaphragm to decouple, i.e., fail to follow the motion of the voice coil. Only the region of the diaphragm in proximity to the voice coil is coupled, and follows the motion of the voice coil faithfully. This results in a decline in a power response of the transducer, especially at higher frequencies.
The decoupling effect has been extensively studied in the art; e.g., William F. Boyce: Hi Fi Stereo Handbook, second edition, October 1964; Abraham B. Cohen: Hi Fi Loudspeakers and Enclosures, revised second edition, 1978. Hence, determining the region of the diaphragm that is not decoupled is a routine engineering task once required design criteria, including, but not being limited to highest frequency of operation, properties of the diaphragm, i.e., shape, size and construction, have been established.
An increased compression ratio may compensate for the decline in the power response; however, the bandwidth of uniform power response is narrowed and generally moved higher in frequency. Furthermore, the above-described decoupling effects are increased.
At least one design attempt, a U.S. Pat. No. 2,832,844, entitled “Speaker Driver”, addressed the above-identified problems of minimizing the air gap and cavity resonances of air gap volume by re-locating the magnetic circuit and the voice coil at the rear side of the phasing plug. However, the effects of uncoupled region and decoupling effects were not solved.
A U.S. Pat. No. 5,177,462, entitled “Phasing Plug for Compression Driver”, expressly rejected the approach taken by U.S. Pat. No. 2,832,844, and instead addressed some of the above-identified problems by creating an “auxiliary air passage” from the air gap. However, adding the auxiliary air passage in the vicinity of the voice coil causes the necessity to replace part of the magnetic circuit with a magnet embedded in the phasing plug. As such, the magnet is necessarily small, resulting in a weaker magnetic field. Furthermore, the air passage itself adds reluctance to the magnetic circuit. To minimize this added reluctance, the auxiliary air passage should take up no more volume than necessary, which compromises the optimal shape and size of the auxiliary air passage.